Emulsion differences using NH4Br vs KBr

[Nodda Duma]

For a non-ammonia-digest Bromo-Iodide or Bromide emulsion, what characteristics of the emulsion change as a result of substituting NH4Br (ammonium bromide) in the make vs. KBr (Potassium Bromide) with no other changes ?

By "substitute" I mean substituting the appropriate amount of NH4Br to maintain the same molar mass of Bromine. For example, if a make recipe calls for 50g of KBr then a proper substitution would be 50g(KBr)*(97.94 / 119.002) = 41.15g of NH4Br, where the molar mass of NH4Br is 97.94 g and molar mass of KBr is 119.002g.

Reduction of peppering is one reason (per Denise Ross). Aside from that, is there a measurable difference in contrast, speed?

-Jason

 

[Photo Engineer]

Yes, there is a difference. Ammonium ion has some solvent capabilities during this precipitation even though it is acidic. It is not as much as a true Ammonia emulsion or Ammonia digest, however, the grains would tend to be more rounded and larger. As a result, speed would be higher and since the grains might tend to be a bit more uniform, contrast would be higher. (In a perfect world...)

Iodide distribution would be more uniform, that is, it would not be concentrated in the core if the kettle starts with Br and I. If I is dumped, there may be no significant difference in the I effect, however it might be hard to detect.

PE

 

[Nodda Duma]

Would you expect a speed loss? In testing I saw a loss of 2 stops of speed when I switched to NH4Br, and I'm wondering if my lab-grade NH4Br is of dubious quality.

 

[Photo Engineer]

You should not lose 2 stops. Did you smell any ammonia over the make while in process? If so, you might need to modify the make. The salt has too much alkali in it.

PE

 

[Nodda Duma]

Did not smell ammonia during the make. I suspect too much Fe, based on what I read of lab grade NH4Br

 

[Photo Engineer]

Peppering can also be reduced by adding a tad more gelatin! And, if that does not work, try some Kodak (Rousselot) gelatin.

PE

 

[dwross]

For a non-ammonia-digest Bromo-Iodide or Bromide emulsion, what characteristics of the emulsion change as a result of substituting NH4Br (ammonium bromide) in the make vs. KBr (Potassium Bromide) with no other changes ?

By "substitute" I mean substituting the appropriate amount of NH4Br to maintain the same molar mass of Bromine. For example, if a make recipe calls for 50g of KBr then a proper substitution would be 50g(KBr)*(97.94 / 119.002) = 41.15g of NH4Br, where the molar mass of NH4Br is 97.94 g and molar mass of KBr is 119.002g.

Reduction of peppering is one reason (per Denise Ross). Aside from that, is there a measurable difference in contrast, speed?

-Jason

 

[dwross]

My experience is that an ammonium bromide emulsion is a little faster, less prone to peppering in minimal precipitation gelatin (i.e. what you want for a fast emulsion), has a little more detail in the shadows and a little less density in the highlights. A KBr negative has a little more "punch," which may sometimes be what's wanted. Of course, there is a certain amount of overlap in characteristics depending on exposure and developer choice.

 

[Photo Engineer]

Thanks for the reminder Denise. We sometimes used mixed salts to kind of combine the virtues of each salt. Sometimes we got the worst of both.

As for peppering, yes, this surprised me too, but use of more gelatin can be the solution.

PE

 

[dwross]

...
As for peppering, yes, this surprised me too, but use of more gelatin can be the solution.
PE

And a good one! My favorite paper emulsion has as much gelatin as I can squeeze into it .

 

[Nodda Duma]

Thanks for confirming what I was suspecting...that my NH4Br might be problematic.

 

[Photo Engineer]

Well, maybe not the chemical itself. IDK for sure Jason. It might just be conditions. NH4Br is OK to use in many cases.

PE